An immunochromatographic assay, also known as a rapid test method, is a method capable of qualitatively and quantitatively analyzing a trace analyte within a short time using an antigen-antibody reaction and has been used to diagnose or detect various diseases and in various fields, including medical, agricultural, livestock, food, military and environmental fields. This immunochromatographic assay is typically performed using either an assay strip comprising a reaction material, which can change in response to the analyte to be detected, or an assay device comprising the assay strip mounted in a plastic case. FIG. 1 is a cross-sectional view of a conventional assay strip which is used in the immunochromatographic assay. As shown in FIG. 1, the conventional assay strip includes: a sample pad for receiving a liquid sample; a conjugate pad containing a conjugate, obtained by conjugating a label, which generates a signal that can be sensed visually or by a sensor to a ligand such as an antigen or an antibody; a porous membrane pad having immobilized thereon a binding substance (antibody or antigen) that binds specifically to an analyte in the sample and/or the conjugate; and an absorbent pad for finally receiving the liquid sample. These functional pads are connected to each other in the above order while partially overlapping each other, and attached to a solid support so that they are continuously arranged. In the case that the assay strip is used in a form being mounted in the plastic case, a sample introduction port for dropping the sample onto a position corresponding to the sample pad is formed on the case, and an observation window for observing assay results is formed at a position corresponding to the binding agent immobilized on the porous membrane pad. In the immunochromatographic assay that is performed using this assay strip, when a liquid sample is dropped onto the sample pad, it moves through the conjugate pad and the porous membrane pad by a capillary phenomenon, and is finally received in the absorbent pad. Herein, the conjugate in the conjugate pad also moves together with the liquid sample, and if the analyte to be detected is present in the sample, the conjugate will bind to the binding agent on the porous membrane pad through the analyte (generally referred to as a “sandwich reaction”), or the conjugate and the analyte competitively bind to the binding agent (generally referred to as “competition reaction”), and thus the presence of the analyte in the sample can be sensed visually or by a sensor.
However, this conventional assay strip has a problem in that the liquid sample that moves by a capillary phenomenon does not uniformly bind to the dry conjugate immobilized on the conjugate pad, and thus there may be a variation between the assay strips, suggesting that the accuracy and reproducibility of the quantitative analysis of the sample can be reduced.
In addition, in a method employing liquid reagents, which is frequently used in biochemical assays, the accuracy and reproducibility of sample analysis can be improved by mixing liquid reagents at a specific ratio in well-plate. However, when liquid reagents including an antibody and an antigen are stored for a long period of time, the affinity of antigen-antibody reactions will be reduced, or the antigen and the antibody will be denatured, suggesting that the liquid reagents are problematic in terms of stability. For this reason, liquid reagents can be cold- or freeze-stored in order to improve their long-term stability, but the cold- or freeze-stored liquid reagents are difficult to use in various measurement sites for point-of-care testing such as rapid testing.
Accordingly, the present inventors have made extensive efforts to increase the stability of a conjugate and improve the accuracy and reproducibility of sample analysis, and as a result, have developed a freeze-dried conjugate bead and a sample pretreatment device including the same, in which the freeze-dried conjugate bead has high long-term storage stability and is allowed to react uniformly with a sample in the sample pretreatment device to provide a reaction product, which has a size and concentration suitable for quantitative analysis and is subjected into an assay device, thereby completing the present invention.